Kassandra is the canon protagonist of Assassin’s Creed Odyssey.
Odyssey lets players choose either Kassandra or Alexios as the playable character at the start of the game. However, later series canon identifies Kassandra as the historical character whose life is represented by the main story. Alexios instead occupies the role of Deimos, the opposing sibling, in that canon presentation.
Kassandra is a Spartan mercenary who travels through the Greek world during the Peloponnesian War. The game is set in the fifth century BCE and includes locations such as Athens, Sparta, and numerous Aegean islands.
The choice system means that players may understandably remember Alexios as their own protagonist. That is valid for an individual playthrough, but when a question asks for the series’ canonical protagonist, the answer is Kassandra. Odyssey was released in 2018.